Many applications require accurate measurement of power radio frequency. These applications include calibrating equipment for radio, television and cellular telephone broadcasting, and field or laboratory applications in which portability is advantageous.
Traditional power measurement compares a known, fixed reference power with the unknown signal, an open loop system to acquire the unknown signal. Prior art open loop power measurement systems require significant signal conditioning to interpret sensing element transfer functions and display absolute, calibrated power accurately. A separate processing unit or meter is required. The very small (microvolt) DC signals from the sensing element must be carried on a signal cable connecting the sensor to the meter. Transmitted low level DC signals are susceptible to drifts, thermocouple voltages, noise and other interference, so some additional signal processing and amplification are performed in the power sensor in close proximity to the sensing element. See I. Braithwaite, "An RF Voltmeter," Ham Radio, pp. 65-75 (November 1987).
Using prior art methods to perform a calibrated power measurement, the power sensor must first be calibrated with the known reference signal (which is usually traceable to a standard) of a fixed frequency and amplitude by physically connecting the sensor to the reference signal port on the power meter. Once the meter is calibrated to that sensor, it can be connected to a device to perform a measurement. Thermal drifts due to ambient temperature changes or self heating of the sensor element are nulled out by "zeroing" the sensor with DC feedback offsets when no applied signal is present.
Zeroing in this manner has significant limitations. When measurements are performed over a variety of power levels, repeated zeroing is necessary to offset errors due to self-heating of the sensing element. On some power meters, this zeroing function can be programmed by a computer, but this is very slow, often taking as long as ten seconds to perform. Since it is often difficult to determine when zeroing is required, users perform this time-consuming operation at the start of a measurement sequence and at times when the device under test (DUT) power level is expected to change significantly. During the zeroing operation it is mandatory that no signal power be present at the sensor input. This may require disconnecting the sensor from the DUT or further disturbing the DUT operating conditions by turning off DC bias voltages if present or using another disruptive means to assure that no power be present at the sensor input.
The signal processing requirements of open loop systems virtually exclude implementation in a modular, single component, portable measurement instrument.